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Warfarin and mechanical valves

I had my aortic valve replaced with a mechanicel valve 3 years ago and have since been on warfarin. It all started well but over the last 3 months my INR has become very erratic and I'm now taking warfarin and injecting tinsiparin. I have an appointment with my GP tomorrow and I'm going to ask about self testing. Is there any help or advice you can give, I have AXA Health insurance.

12 September 2019

This content was last reviewed in September 2019 by Hema Love, a registered pharmacist in our Health at Hand team.

Thank you for contacting Health at hand.

Warfarin is an anticoagulant that has been successfully used in the UK for many years.

It is used in many conditions where there is a high risk of blood clot, so it is used to prevent these clots from occurring. Deep veen thrombosis, pulmonary embolism, atrial fibrillation and heart valve replacements are just a few such conditions that increase your risk of developing a blood clot.

How does warfarin work?

Warfarin stops the action of Vitamin K.

When you cut yourself the body plugs the cut with a blood clot to form a scab. 

When the blood starts to clot platelets in the blood clump together and trigger a set of complex chain reactions known as the coagulation cascade, a bit like a domino effect. 

Blood proteins are known as coagulation factors or clotting factors act to stop a bleed. These clotting factors are mainly made in the liver and they require vitamin K for these changes to occur. 

Warfarin blocks the action of vitamin K in the liver so increases the time to form a clot. Warfarin prolongs the bleeding time.

Prothrombin time is a blood test that measures of how long it takes for the blood to form a clot.

Warfarin prevents existing clots getting bigger and new clots from forming thereby preventing strokes, heart attacks and clot in the lungs.

Of course uncontrolled bleeding can be fatal so the amount of anticoagulation that is required needs to be monitored carefully. The intensity of anticoagulation is expressed as INR.

What is INR?

INR is an abbreviation of the International Normalised Ratio. INR is a measure of how long it takes a given amount of blood to clot for a person who is taking warfarin compared to how long it takes for the same amount of blood of an average person who is not on warfarin. If your INR is 3 then it takes three times as long for your blood to clot then an average person.

In other words INR is the prothrombin time expressed as a ratio of the clotting time for patient plasma divided by the clotting time for control plasma corrected by a standardising factor. The higher the INR the greater the risk of bleeding. If the INR of a person who is on warfarin is too low then the greater the chance of a clot forming. This is why it is important for a person on warfarin to be within the target INR limits.

The amount of warfarin you take will be adjusted according to the INR with the aim to keep your INR as close to the target level as possible.

If your INR is lower than your target level then your dose of warfarin will be increased and adjusted until your INR reaches the target level.

If your INR is lower than your target level it means that you are more at risk of forming a clot. 

If your INR is higher than required the clinic will reduce your dose of warfarin and monitor your INR more closely. If it is very high then you may be given vitamin K to bring it down. Again your INR will be closely monitored.

If your INR is higher than your target level it means that you are more at risk of haemorrhage.

Why are my INR readings erratic?

There are many factors that can affect your INR readings and these include;

  • Changes in diet

Vegetables such as spinach, broccoli, Brussels sprouts, avocado and other vegetables and fruits such as berries and figs have a large amount of vitamin K. If you keep drastically changing the amount of these vegetables you eat you will vary the amount of vitamin K that is in your body. Since warfarin blocks the action of vitamin K you will need different amount of warfarin to cope with the changing amount of vitamin K in your body. If you keep to the same amount of vegetables everyday you should have a stable INR.

  • Changes in alcohol consumption

You can drink alcohol whilst you are on warfarin as long as you drink in moderation and keep to within the limits set by the NHS. They recommend 14 units per week for men and women.

  • Drinking cranberry juice
  • Medications

Some medications including some antibiotics can affect your INR, causing it to go up or down. Your anticoagulation booklet with tell you that you cannot take non-steroidal anti-inflammatory NSAIDs, such as ibuprofen so you know you should not have these. However, what you may not be aware of is that sometimes paracetamol can also affect your INR, so always check with your pharmacist. If you are on any new medication inform your pharmacist, doctor or healthcare professional at the anticoagulant clinic.

  • Herbal medications and over the counter medications and supplements

Ginko biloba may increase your risk of bleeding. St John’s Wort, ginseng and garlic tablets may reduce blood levels of warfarin. Before you start taking any herbal supplement or any over the counter medicines or supplements ,always check with your pharmacist, doctor or healthcare professional.

If any of the above factors apply to you then this could account for your erratic INR results. Of course if you are taking any medicines or food or drinks listed above then you will need to take advice from your GP, nurse, pharmacist or other healthcare provider before you stop.

What is tinzaparin?

Tinzaparin is low molecular weight heparin LMWH. LMWH are injections that vary in strength like other medicines the dose is specific to each patient based on their weight and requirement.

How does it work?

LMWH enhance the effect of certain coagulation factors thereby prevent clots. They are given by injection into fat usually in the tummy once or twice daily. Tinzaparin may lower platelets so it is not given on a long-term basis.

All LMWH are derived from pork. People who cannot use pork derived products due to religious reasons will need alternative anticoagulant called Fondaparinux.

Newer anticoagulant agents NOACs

Recently dabagatrin, rivaroxaban, apixaban and endoxaban have been introduced as alternatives to warfarin. They are also known collectively as NOACs. However they are not licensed to be used to prevents clots if you have mechanical valves.

INR self-testing

I can understand why you feel that checking your INR yourself may be a lot more convenient for you.

There are a number of self-testing devices on the market that are particularly useful for someone in your situation.

They are hand-held easy to use devices that you can use at your own convenience and avoids frequent visits to the anticoagulant clinics and time off work.

You will need to talk to your GP about your option of self-testing. The device itself is not available on an NHS prescription but the strips that are used with the device are.

You will need to buy a device and there are several devices to choose from and you will need to be trained in using the device. 

How to measure your INR with a self-monitoring device

  1. First turn on the device and place a test strip into the device.
  2. Next, you'll need to prick your finger with a lancet pen: Load the pen with a lancet then place the pen over your clean fingertip and push the plunger. You will feel a little prick
  3. Gently press your finger-tip to draw a small drop of blood.
  4. Place the drop of blood onto the designated area of the test strip.
  5. Wait a while and the device will give you a reading.

Of course there are many other functions available on the device and data can be downloaded onto a desk top. See the web links below for further details.

Warfarin and self-monitoring - Anticoagulation UK

CoaguChek® INRange system - Roche

Further information

The websites below will provide more information:

Warfarin - NHS patient information

Food guide for people taking warfarin - Royal Surrey NHS

Calculating alcohol units - NHS Live well

UK alcohol unit guidance: CMOs' Low Risk Drinking Guidelines - Drinkaware

Unit calculator - Drinkaware

We think you should wait and see what your doctor has to say tomorrow. Please do not hesitate to contact us if you need further information.

Answered by the Health at Hand team.

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